Resolving Multicache's

[laddie-tude]

I was recently in reading and did find UniCache which involved measuring 700 metres from Jeffs Birthday box, 500 metres from Earley Bird and 100 metres from a given long/lat ref. I solved it by charting it on a O/S map but is there a way to do it with my etrex GPSr?

 

Thanks

[+Pengy&Tigger]

Nope, your choices are:

 

1) convert co-ords to British grid and solve simultaneous equations

2) plot on a map (paper or electronic)

 

You picked the easier option!

 

T

Edited November 4, 2004 by Pengy&Tigger

[+M3ZPY]

There is another way, just download the waypoints from each cache and upload them to your gps unit then transfer the waypoints on Mapsource from your GPs and measure the distance using the ruler tool. Here endeth the lesson.

[+paul.blitz]

Heck, there's a FAR simpler solution:

 

Use THREE GPSr units, each with one of the 3 waypoints. Then wander around until the 3 units show the correct distances (ie manual solving of a simultaneous equations).

 

Paul

[+Pengy&Tigger]

There is another way, just download the waypoints from each cache and upload them to your gps unit then transfer the waypoints on Mapsource from your GPs and measure the distance using the ruler tool. Here endeth the lesson.

That's what I meant by 'plot on map - paper or electronic'

[+Pengy&Tigger]

Heck, there's a FAR simpler solution:

 

Use THREE GPSr units, each with one of the 3 waypoints. Then wander around until the 3 units show the correct distances (ie manual solving of a simultaneous equations).

 

Paul

In some cases the distances are several miles. You would at least need to know roughly the correct co-ords, or it would take hours/days just wandering randomly!

 

T

[+The Forester]

Tigger's right. So were you, laddie.

 

Your graphical method was the simplest and most traditional way to solve the trilateration problem.

 

Given three lines of position, as you were, it should also be possible to ascertain the accuracy of the given position fix. If there is a great big area within the 'cocked hat' of the intersecting arcs of your geometrical compass-drawn lines, then there is probably a large aera of uncertainty about the quality of the fix. If the three lines of position intersect very closely then either the fix has been post-processed or it is a tight fix (or perhaps both).

 

The error indication shown on some GPSrs uses a similar method to estimate the quality of a displayed fix. It shows the standard deviation or standard error of the residuals of the pseudoranges of all the NavStar satellites used in the displayed fix. It merely does what you did on paper mathematically, by solving the simultaneous equations which Tigger mentioned.

 

Cheers, The Forester

[+Team Ullium]

I must admit that Tigger's simultaneous equation suggestion seems not only the simplest ... but probably the most accurate (exchanging to OS co-ords first of course!)

 

Ullium.

[+The Forester]

For spritely and clever people, like Ullium and Tigger who can invert a variance-covariance matrix in their head to produce a Least Squares solution, it is a simple algorithm and has the advantage of being quantifiably very accurate.

 

For short-sighted old fogeys like The Forester, beam compasses are easier and more convenient to wield on a paper map!

 

 

Cheers, The Forester

least squared residual

 

Old hydrographers used the cosine rule to match pairs and just meaned the lot, but perhaps that's why they are no longer with us!

[+Birders]

We did this cache the easy way.. MM allows you to draw a circle around any cache.. simply draw circles of the appropriate radius around each point and where they intersect - Bingo!!

[+Pengy&Tigger]

For spritely and clever people, like Ullium and Tigger who can invert a variance-covariance matrix in their head...

Or who can set up a spreadsheet in a couple of minutes after having dug out an A level (or equivalent!) maths textbook!

 

T

[AJK]

For the excel way of doing this, you really have to switch to a different coordinate system, rather than lat long.

 

In the U.S. we switch to UTM, UK would be the OS grid I assume.

 

In some cases, the PC mapping method may be more accurate - especially if the hider used the same method.

 

With equations, you may not find an exact solution. However, I have a workbook that I use that seems to work most of the time.

 

You can (just about) do this with one GPSr, depending on the model. On my Legend, I can go to the nearest waypoints screen, and as I scroll through the waypoints, it shows the distance from the current spot. I've never tried doing this though - I'll stick with maths.

[+Team Ullium]

For spritely and clever people, like Ullium and Tigger who can invert a variance-covariance matrix in their head to produce a Least Squares solution, it is a simple algorithm and has the advantage of being quantifiably very accurate.

 

Speaking purely for myself...I wouldn't describe myself as spritely nor clever...dotage comes to many of us in the end

 

But if you can understand Matrices it certainly makes the maths even simpler!

Couple that to The Trachtenberg Speed System of mental arithmatic and you can probably throw away your computer and maybe even the GPSr...no that's going a bit far

 

Ullium.

[+Billy Twigger]

You could also use The Force. Works for me!

 

If only I could get a decent signal!!!!

[laddie-tude]

We did this cache the easy way.. MM allows you to draw a circle around any cache.. simply draw circles of the appropriate radius around each point and where they intersect - Bingo!!

Hi Birders,

I recognise your name from various caches in the Reading area. We did as you say but on a paper OS map.

 

Can't remember, have you done th Bill and Ben Cache - fabulous walk and when we did it we saw a dozen parotts - no really. But then at the same time we saw two red kites - it was incredible, who needs Beale Park?

[+Team Ullium]

You could also use The Force. Works for me!

 

If only I could get a decent signal!!!!

Which force would that be BT?

 

Force of circumstance? or Force Majeure?

 

 

Ullium.

[+Pengy&Tigger]

We did this cache the easy way.. MM allows you to draw a circle around any cache.. simply draw circles of the appropriate radius around each point and where they intersect - Bingo!!

That's what I would probably do on that particular cache. The one I have experience of is Circumferentially challenged where you have to find the waypoints of local multis then determine the centre of the circle where the points lie on the circumference of a circle. Drawing circles on maps could lead to some big errors on this one as the points are quite close together on the circumference. Maths was definately the way to go on this one as you have no distance to go by.

 

T

[+Bob Smith]

I took the OS co-ords and used the 'solver' plug in for Excel to find the cache location for the Uni cache. Result was spot on.

 

Bob

[+welch]

Heck, there's a FAR simpler solution:

 

Use THREE GPSr units, each with one of the 3 waypoints. Then wander around until the 3 units show the correct distances (ie manual solving of a simultaneous equations).

 

Paul

'Moe, Larry, and Curly go GPS!'

[+Pharisee]

Another way, and one that I've used successfully, is to use a CAD package (AutoCAD, in my case). Convert the lat & long to an OS grid ref, use that as the X & Y co-ordinates to plot a 'point'. Draw a circle of the required radius centred on the 'points' and then read off the X & Y co-ordinates of the intersection. Treat them as an OS grid ref and if you want, convert it back to lat & long.

[Red Panda]

Edited November 7, 2004 by Red Panda

[+Billy Twigger]

You could also use The Force. Works for me!

 

If only I could get a decent signal!!!!

Which force would that be BT?

 

Force of circumstance? or Force Majeure?

 

 

Ullium.

Looking at the pic again.................it would need to be force of habit!